1. Field of the Invention
The present invention relates to a method of writing information into a fuse-type read only memory (ROM), and more particularly to a method of electrically blowing (i.e., opening) a fuse formed in a memory device for programming.
2. Description of the Prior Art
Semiconductor memory devices provided with fuses include a fuse-type ROM and a random access memory (RAM) with redundant circuits. In a fuse-type ROM (i.e., a fuse-link-programmable ROM), the fuses are selectively blown in order to write information, and in a RAM with redundant circuits, the fuses are selectively blown in order to replace a row or column containing a bad bit (i.e., defective memory cell) with a spare row or column: the replaced portion of the RAM can be regarded as a ROM portion. The fuse blowout is achieved by feeding an excess current and voltage to the fuse.
In general, a fuse in the memory device is made of a fusible material, e.g., polycrystalline silicon, is formed on an insulating film, and is covered with a protective film, e.g., a phosphosilicate glass (PSG) film (reference example, cf. David W. Greve, "Programming Mechanism of Polysilicon Resistor Fuses", IEEE Transactions on Electron Devices, vol. ED-29, No. 4, April 1982, pp. 719 to 724, particularly, FIG. 1b). Usually the fuse has the shape of a bow-tie, i.e., a long narrow center portion and two wider pad portions. The two pad portions are joined by the center portion and are connected to conductor lines, e.g., aluminum lines, respectively.
The fuse is blown by feeding it an excess current, and a pulse voltage is adopted as the applied voltage. When the fuse is blown, it bursts in such a manner that the protective PSG film is also broken, forming an opening in the film. This opening must be filled by re-forming the protective PSG film to prevent contaminants from entering the memory cell.